Determination of Potassium Levels in Intensive Subsistence Agricultural Soils in Nyamira County, Kenya

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Assessment of potassium levels in agricultural soils ofNyamira County, Kenya wasnecessitated by the observed progressive drops in maize acreage yields over the years despite use of phosphorus and nitrogenousfertilizers. In the study,concentration levels of potassium and other soil fertility indices such as, organic carbon, cation exchange capacity, exchangeable cations, soil pH, available nitrogen, total and available phosphorus were determined. Five composite soil samples were collected at depths of between 0-30 cm from five farms that have consistently been under intensive cultivation. Fractionation of potassium was achieved by sequential extraction of soil sampleswith distilled water, ammonium acetate and nitric acid in that order. Concentration levels of potassium in the extracts were determined using a flame photometer. Potassium concentration levels obtained from the water soluble soil extracts were used to calculate thermodynamic parameters such as free energy of replacement, potassium activity ratio and ionic strength of the soil solution.The relationship between the adsorbed and equilibrium potassium concentration, quantity/intensity was determined by plotting Freundlich adsorption isotherms.The isotherm was used todeterminethe buffering capacity ofpotassium and the concentration levels of potassium adsorbed on un-specific sites in the soil. The suitability of the adsorption equation was determined by applying the least square regression analysis.From the study available potassium in the soilsranged from 57 to 70 mg/kg and had amean value of 60±5.54.2 mg/kg (ammonium acetate method). The water soluble potassium ranged from 1.8 to 2.2 mg/kg with a mean of 2.02±0.16 mg/kg. Nitric acid extracted potassium had a mean of 149.±2.306 mg/kg. The mean free energy of replacement,∆F,was found to be -3572±44.98 cal/molindicating that the soils have low supplying power of potassium. The potassium buffering capacity of the soils was found to have a mean of 1.189±0.06 mg/kg. The amount of potassium adsorbed on un-specific sites of the soilshad a mean value of 6.993±2.378 L/kg. These findingsreveal the extent of potassium depletionin the soils ofthisregion andwill form a baseline for working acreage potassium doses required for remediation.
Key words: potassium buffering capacity, adsorption, Freundlich isotherm, thermodynamics parameters, free energy of replacement, soil fertility indices